Dental cements have been undergoing continuous developments since before the use of powdered zinc oxide mixed with eugenol about a century ago. The important characteristics which a dental composite must exhibit include: adequate mechanical properties such as strength, hardness, smoothness and abrasion resistance; optical characteristics that permit a simulation of the tooth such as color matching, color stability and a matching index of refraction; and a variety of more complex characteristics which assure compatibility with the human body in an oral environment such as non-toxicity, non-solubility, low water absorption and radiopacity to name a few.
Early successful composite resin dental restorations for anterior implacement were prepared by Bowen in 1963. He used a silane-coated fused quartz powder as filler. In 1969 Chang recommended the use of fillers approximating the refractive index of the resin matrix consisting of glass beads and subordinate amounts of glass fibers and lithium aluminum silicate powder. Chang states that glass beads used as filler can range anywhere from 5 to 100 microns in diameter. Lithium aluminum silicate powder is used by Chang as a fine-grained component and contains both spodumene and .beta. eucryptite to give low thermal expansion characteristics to the composite. See Bowen, Properties of a Silica-reinforced Polymer for Dental Restorations, 66 J ADA 57-64 (January 1963); CHANG, U.S. Pat. No. 3,452,437.
LEE II, U.S. Pat. No. 3,539,533 states that aluminum oxide may be used as a filler but recognizes that its high refractive index (&gt;1.75) gives unsatisfactory translucency for anterior restorations. He found that mixtures of quartz and glass beads are satisfactory esthetically as fillers. His contention is that the quartz must be ground through 200 mesh U.S. sieves.
In the patents of both CHANG and LEE II, and in all subsequent patents surveyed, the filler is given a silane coating. Commonly the silane is carried on to the filler with aqueous solutions; e.g., LEE II uses dilute acetic acid silane solutions.
WALLER, U.S. Pat. No. 3,629,187 recommends the use of both ceramic and glass fillers less than 25 microns in diameter. He claims that the use of mixtures of ceramic and glass fillers allows for the preparation of a good particlesize distribution of the filler fraction so that the composite product may contain both fines and particles nearing the maximum diameter of 25 microns. In WALLER, the glasses that can be used may be phosphate or silica. The ceramic filler that he particularly recommends using is lithium aluminum silicate powder with a view to lowering the thermal expansion and is probably a major component of the fines.
GANDER, U.S. Pat. No. 3,835,090 states that representative fillers utilized for restorative resin composites are quartz, glass beads, aluminum oxide, and fused quartz, or silica. The inventor states that a preferable size range is 25-30 .mu.m. Mica, glass fiber, and nylon filled plastic composites have been evaluated by others.
As a further improvement in the fillers used in restorative resin composites, ROSSI, U.S. Pat. No. 3,792,531 discloses filler systems which are stated to give improved polishability (finishability). These filler systems utilize fine crystalline quartz with diameters between 0.70 and 30 .mu.m. Eighty percent of the quartz particles by weight are to be less than 20 .mu.m in diameter and 20 percent less than 5 .mu.m in diameter. ROSSI states that larger particles are loosely held by the resin and therefore are susceptible to being gouged out by routine polishing techniques.
Filler materials having low thermal expansions have been recommended by BOYD, U.S. Pat. No. 3,503,128. That patent suggests topaz, 4.7.times.10.sup.-6 /.degree.C.; alumina, 8.7.times.10.sup.-6 /.degree.C.; zirconium orthosilicate (zircon), 4.2.times.10.sup.-6 /.degree.C.; and white beryl, -1.35.times.10.sup.-6 /.degree.C. All, with the exception of white beryl, have refractive indices which are too high to yield anterior restorations having satisfactory translucencies. The lowest thermal expansion composites disclosed in that patent has an expansion of 19.1 to 27.9.times.10.sup.-6 /.degree.C. over a temperature range 0 to 60.degree. C. utilizing .beta. eucryptite in major amounts.
X-ray opacifying fillers were first introduced in glass form by Bowen and Cleek. See Bowen and Cleek, X-ray-Opaque Reinforcing Fillers for Composite Materials, 48 J. Dental Research, No. 1 (January-February 1969). These glasses contained 24 to 28 mole percent of barium fluoride plus oxide. A glass composition which was considered particularly favorable contained in mole percent SiO.sub.2, 44; BaF.sub.2, 28; B.sub.2 O.sub.3, 16; and Al.sub.2 O.sub.3, 12; or in weight percent SiO.sub.2, 26.72; BaF.sub.2, 49.64; B.sub.2 O.sub.3, 11.27; and Al.sub.2 O.sub.3, 12.37.
Another set of radiopacifying glass was later developed by Bowen and Cleek. See Bowen and Cleek, A New Series of X-ray-Opaque Reinforcing Fillers for Composite Materials, 51 J. Dental Research, No. 1 (January-February 1972). They could not obtain glasses with refractive indices less than 1.592 which contained more than 7 mole percent ZrO.sub.2. Also, these glasses were heterogeneous and had only 7 mole percent of BaO. In weight percent these maximum zirconium-containing glasses had ZrO.sub.2, 11.7, and BaO, 14.5. In their later glasses a particularly favorable composition contained in mole percent SiO.sub.2, 66; BaO, 17; B.sub.2 O.sub.3, 6; or in weight percent, SiO.sub.2, 49.87; BaO, 32.80; B.sub.2 O.sub.3, 9.64; and Al.sub.2 O.sub.3, 7.69.
CHANDLER was the first to apply and evaluate Bowen's radiopaque glass-resin composites. See Chandler, Bowen and Paffenbarger, Clinical Evaluation of a Radiopaque Composite Restorative Material After Three and a Half Years, 52 J. Dental Research, 1128-1137, No. 5 (1973). Anterior restorations were prepared utilizing approximately 33 wt % of a Corning BaF.sub.2 glass and 66.7 wt % of a Corning fused silica in the filler fraction.
Later DIETZ, U.S. Pat. No. 3,826,778 suggested barium aluminum silicate glasses which contain greater than 22.5 wt % BaO. It stated that the optimum radiopacifying glass consists of the following weight percentages: SiO.sub.2, 25-35; Al.sub.2 O.sub.3, 10-20; and BaO, 50-60.
MULLER has developed La.sub.2 O.sub.3 containing glass-ceramics with a high radiopacifying powder. See Muller, Glass Ceramics as Composite Fillers, 53 J. Dental Research, 1342-1345, No. 6 (November-December 1974). These contain up to 15 wt % La.sub.2 O.sub.3. However, the Vicker's microhardness of these fillers is high, 950 kg/mm.sup.2, and is probably objectionable in finishing.
A few of the many additional patents and printed publications which investigate metal oxides, silica, quartz, mica, ground glass, and other materials for use as fillers in dental composites include: Muller, U.S. Pat. No. 3,973,972; Mabie, U.S. Pat. No. 3,973,970; Jurecic, U.S. Pat. No. 3,971,754; Dietz, U.S. Pat. No. 3,801,344; Overhults, U.S. Pat. No. 3,839,065; Wilson et al., U.S. Pat. No. 3,814,717; Brigham, U.S. Pat. No. 3,649,732; Siegel, U.S. Pat. No. 3,504,437; and Saffir, U.S. Pat. No. 3,069,773.
Fillers made according to the prior art have many disadvantages which it is the object of this invention to overcome. Glass beads have been observed commonly to be subject to extensive plucking out on finishing. Quartz fillers, except when extremely fine, degrade under impact finishing or else remain high above the resin matrix when the finishing becomes worn.
Typical quartz and glass bead fillers give very rough finishes. A better finish using quartz-filled composites may be obtained only if the particle size of the quartz filler is reduced to a point where it raises the BIS-GMA paste viscosity excessively. This reduces the amount of filler than can be introduced into any given resin system.
Fillers using fused silica, spherical glass, quartz, ceramic and alumina generally do not have sufficient radiopacity to permit ready determination of decay under the composite restoration.
In order to delineate the composite restorations against both dentin and enamel, x-ray opaque materials were developed, but these glasses commonly contained considerable amounts of potentially soluble and possibly toxic constituents, e.g., BaF and BaO which can have an adverse effect on the nervous system. Futhermore, barium containing glass beads tend to severely degrade under the impact of finishing wheels.
Fillers having adequate radiopacity can be prepared using strontium, but the large mole percentage of strontium required will probably result in high solubility. Glass ceramics using La.sub.2 O.sub.3 as an x-ray opacifier have also been investigated.
Some attempts to improve the finish of composites have focused on reducing the plucking of glass filler by etching the surface. Still other attempts have improved finish by using finer grained quartz.
In order to meet the objections of possible soluble toxic constituents and to improve finish and x-ray opacity, the microporous glass fillers of this invention have been developed. Improved finish is due, at least in part, to filler absorption of the impact of dental finishing wheels. Therefore, shock is not transmitted to the filler-grain resin interface. Also these glasses give systemically nontoxic x-ray opacification.